1. Field of the Invention
The invention relates to a screw hole in furniture fittings, especially in metal hardware for furniture, in which a screw provided with a truncoconical countersink head and having a thread of a given outside diameter is preinstalled, wherein the hole has a countersink shaped to complement the truncoconical head of the screw and adjoined by a substantially cylindrical section to accommodate the shaft of the screw. The invention also concerns a method for the production of such holes in hardware made of thin sheet metal.
In hardware that is to be fastened by screws to pieces of furniture, e.g., mounting plates for furniture hinges, it is common practice to preinstall the screws on the hardware part, so that when the latter is mounted on a cabinet wail the hardware part needs only to be placed at the intended mounting position with its preinstalled screws aligned with the holes predrilled in the furniture and the screws to be driven in with a screwdriver. To achieve sufficient strength in the screw connection, screws with relatively coarse threads are used, so-called "Euro screws" being increasingly used of late, the threads of which have a relatively coarse crest, which on the one hand permits the screws to be driven with comparatively little effort into holes in wood materials, but due to the comparatively deep penetration of the threads into the walls of the hole they can withstand heavier loading and still permit repeated installation and removal of the hardware.
To assure that the preinstalled screws will be held securely in the preinstallation position, i.e., will neither drop out nor tilt away from their position at right angles to the hardware surface, a relatively tight fit must be provided between the top end of the screw's shaft and the hole.
This has been accomplished in the case of thick hardware (DE-PS 34 44 851) by making the wall of the hole holding the shaft of the screw slightly smaller than the outside diameter of the shaft of the screw, so that the crests of the threads can easily penetrate into the wall of the hole during preinstallation and cut a shallow holding thread by themselves, so to speak.
2. The Prior Art
In the case of hardware of thin sheet metal, in which the hole thus has such a short length that a thread holding the screw securely can hardly be produced, a solution to this has been found (DE-PS 36 22 001) in which a circular area extending over about 270.degree. is stamped out of the area of material adjoining the hole or, more correctly, the punched hole, and shaped out of the plane of the sheet metal to correspond to the pitch of the screw thread. Thus a thread is formed which suffices to hold the screw, especially above-mentioned Euro screw, securely in the punched hole. Of course, free space must be available to correspond to the thread formed out of the plane of the material, i.e., sheet-metal hardware of this kind must be embossed at least in the area of the punched fastening holes to form the necessary hollow space. When the screws are driven into the corresponding holes, the unthreaded section at the upper end of the screw shaft adjoining the screw head and having a diameter smaller than the outside diameter of the threads comes into the reach of the punched thread and the screw can then turn freely without binding, i.e., the piece can be screwed tightly onto the corresponding cabinet wall in the required manner.
The invention is addressed to the problem of devising a configuration for screw holes in hardware parts which will permit secure preinstallation of flat-head screws, and which on the other hand will be usable both in hardware of great material thickness and especially in hardware of thin sheet metal, while it will be possible for the part in question to be forced with its bottom into tight contact with the cabinet wall without routing out the area around the mounting hole. At the same time the hardware piece provided with the holes in question is also to be installable even without preassembly and with screws of different threading or smaller-shaft diameter if their heads fit into the countersink around the hole.
Setting out from a hole of the kind described above, this problem is solved in accordance with the invention in that the inside diameter of the cylindrical section of the hole is slightly smaller than the outside diameter of the screw shaft measured across its threads, and in that a thread is provided in the cylindrical section of the hole to accommodate the thread of the screw shaft, and has a length that is shorter than the unthreaded portion of the shaft adjoining the screw head. Hardware with holes thus configured can, of course, also be installed without a preinstalled, mating screw, so that flat-head screws with nonconforming shafts, e.g., of different screw pitch, different size can be used, as long as it is certain that the head will fit into the countersink around the hole. Then, if screws with an outside thread diameter that is smaller than the inside diameter of the threaded section of the hole are used, screws can also be used whose thread runs over the entire length of their shaft, i.e., reach all the way to their heads.
The root diameter of the thread in the cylindrical section of the hole can be less than the crest diameter of the threaded shaft, in which case it is recommendable to configure the thread of the cylindrical section of the hole so that the threads of the pre-installed screw are held in the hole with a light force fit.
The end of the substantially cylindrical, threaded hole section remote from the countersunk portion is expediently adjoined by a short hole section of expanding diameter, whose diameter at the bottom of the hardware piece facing away from the countersunk portion is approximately equal to or slightly larger than the outside diameter of the screw measured across the threads.
In hardware of thin sheet metal, the fastening hole is produced in accordance with the invention such that, in the flat sheet metal blank, first a hole of small diameter is punched, and the area of the sheet metal adjoining the hole is then forced upward to form a substantially cylindrical projection protruding above the upper side of the metal blank, and having an inside diameter substantially equal to the inside diameter of what will later be the substantially cylindrical hole section, and then at least a portion of the cylindrical projection raised above the upper surface of the metal blank is flared by means of a punch with a conical face pressed into it, and then finally the thread is produced in the remaining cylindrical hole section.
The thread can be made either by a noncutting method or it can be cut into the cylindrical hole section.